The invention relates to an apparatus for regulating a parameter, such as a pH value of a medium, by admitting an additive into a medium, with a control signal generator which is dependent upon the departure from norm of the medium and with a regulating element which effects the admission of additive for the duration of the control signal.
In a conventional apparatus of such character (refer to German Pat. No. 2,641,101), control signals are obtained by use of a timing capacitor which is charged by way of a resistor in dependency on the extent of deviation from the norm and is discharged by way of a threshold value switch. The discharge impulses control a timer which transmits control signals of constant width but of variable frequency. An electromagnetically operated metering pump receives rectified alternating current voltage for the duration of each control signal. The metered quantity which is delivered per unit of time decreases with decreasing deviation from the norm.
In such a regulating device, the frequency of control signals can be varied between 10 and 100 signals per minute. Thus, the metered quantities can be regulated at a ratio of 1:10. The minimum duration of a period is approximately six seconds and the maximum signal width is approximately 0.6 seconds. This necessitates the carrying out of switching operations at a very high frequency. If the regulating element is a solenoid-operated valve, the desired metered quantity of material cannot pass the valve within such short intervals of time.